Hi,
Given that each multi-bridge connection, when the Stored Process Server is fully loaded, will result in a SAS process I think it boils down to how many SAS stored process server processes is appropriate for your expected load, your expected response time, and what your hardware can handle, giving consideration to what with other processes may also need to run on that machine (or set of machines). There are a variety of tuning parameters that can be used to control load balanced servers and you also have the option to scale out with a load balanced cluster of multiple machines if necessary. You can read more about it in the Overview of Load Balancing section in the SAS 9.3 Intelligence Platform: Application Server Administration Guide.
In your instance, 5 connections does not sound like too many (however I don't know the specs for your hardware and don't know what those stored processes are doing). There is an interesting discussion of stored process server load balancing and tuning parameters in SAS Usage Note 40567: Troubleshooting and tuning the SAS® Stored Process Server and SAS Pooled Works... Whilst this usage note relates to a SAS Marketing Automation situation, it does mention "... At least 9 is recommended ..." and "... if you have defined 40 Multibridge Stored Process Server connections..." so this might give you some examples of the numbers that have been used by others.
You could also try to set up a test environment, with a variety of multi-bridge connections and tuning parameters, run a simulated load of expected stored processes and find the sweet spot for response times. In the past I have used Apache JMeter as software to generate a simulated concurrent client load for a SAS Stored Process Server.
I hope this helps.
Cheers
Paul